Cleanup async flag and use feature flag for debugging (#51962)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/51962

Changelog: [Internal] - Migrate debug feature flag to be accessed in both native and JS

Reviewed By: yungsters, mdvacca

Differential Revision: D76381273

fbshipit-source-id: d4071abeb9769821e236c444f89044165cf83d92